An internal combustion engine may operate in a variety of different conditions, particularly in modern engine systems that are electronically controlled based upon a variety of monitored engine operating parameters. In each operating mode it is not uncommon to use different techniques to determine the amount of fuel to deliver to the engine for a fuel delivery cycle. For example, different fuel rate maps might be utilized in two different modes, or a fuel rate map may be used in one mode and an engine speed closed-loop control may be used in another mode.
Automotive vehicles commonly employ a port-injected internal combustion engine in which a fuel injector sprays fuel into air in an intake manifold of the engine near an intake valve of a cylinder as air gets pulled into the cylinder during the cylinder's intake stroke. The conventional fuel injector is typically controlled in response to a fuel injection pulsewidth signal in which the pulsewidth determines the amount of fuel injected into the corresponding cylinder of the engine. The fuel injection pulsewidth signal can be implemented to follow a programmed target fuel injection curve. The programmed target fuel injection curve determines the fuel injection pulsewidth and is generally utilized to provide adequate engine performance when feedback engine control is not available.
Many automotive vehicles commonly employ an oxygen (O.sub.2) sensor generally disposed upstream of the exhaust system for sensing the oxygen level in the exhaust gas emitted from the engine. The oxygen sensor can serve to provide a feedback signal to control engine operation and adjust fuel injection to the engine to achieve better engine performance. Some conventional oxygen sensors, however, are required to warm up to a sufficiently high temperature before an accurate oxygen sensor reading may be obtained. Also, following an engine start, the oxygen sensor and processing devices initially may not have acquired enough information to provide adequate feedback control. Therefore, for a period of time immediately following cold start up of the vehicle engine, the oxygen sensor may not be capable of providing accurate information with which the engine may be controlled to operate to achieve low hydrocarbon emissions. As a consequence, excessive hydrocarbon emissions may be emitted from the vehicle within the immediate period following start up of the engine.
Additionally, immediately following a cold engine start, the catalyst of the catalytic converter can be ineffective because the catalyst requires a period of time to warm up to a temperature at which the catalyst can operate effectively to burn excess hydrocarbons. As a consequence, hydrocarbon emissions may initially be high due to poor burning of the excess hydrocarbons due to a low temperature catalyst. Adding to the problem, an over abundance of fuel in the catalyst may further cool the catalyst, thereby requiring an extended period of time for the catalyst to warm up to a sufficient operating temperature.
One approach for modifying fuel injection to the engine is described in U.S. Pat. No. 5,492,102, entitled "Method of Throttle Fuel Lean-Out for Internal Combustion Engines", issued to Thomas et al. on Feb. 20, 1996. The aforementioned issued U.S. patent is incorporated herein by reference. The approach described in the above-identified issued patent calculates a fuel lean-out multiplier value that is applied to a fuel pulsewidth value of the fuel injectors to reduce the amount of fuel injected into the engine by the fuel injectors. In the aforementioned approach, the fuel lean-out multiplier value is determined from a sensed throttle position and sensed deceleration.
It has also become increasing desirable to operate an engine lean in order to improve fuel efficiency and meet emissions standards. This is accomplished by adjusting the air/fuel ratio, which, in internal combustion engine design, is typically considered to be the ratio of the mass flow rate of air to the mass flow rate of fuel inducted by the engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If too much fuel is being burned in proportion to the amount of air to achieve perfect combustion, the air/fuel ratio is less than stoichiometric and the engine is said to be operating rich. Manipulating the air/fuel ratio to rich is typically advantageous in achieving maximum power such as during acceleration of an automobile. Similarly, if too much air is being burned in proportion to the amount of fuel to achieve perfect combustion, the air/fuel ratio is greater than stoichiometric, and the engine is said to be operating lean. Operating the engine lean is typically advantageous in achieving fuel savings when maximum power is not needed. Operating the engine lean furthermore provides the advantage of reduced cylinder head temperatures.
Conventionally, immediately after the start of the engine, the air/fuel ratio supplied to the engine is controlled to a value richer than a stoichiometric air/fuel ratio in order to ensure stability of rotation of the engine. The ability to maintain the required stability of rotation of the engine even when the air/fuel ratio is controlled to a leaner value than the stoichiometric air/fuel ratio would realize the fuel efficiency and emissions benefits of lean engine operation.
But controlling the air/fuel ratio of the mixture to a leaner value than the stoichiometric air/fuel ratio is not possible for all operating conditions of the engine. For example, when the engine temperature is higher than an expected value, fuel vapor can be generated in the fuel supply line causing problems of low stability of rotation of the engine, and even engine stalling. Also, upon a sudden change in the air/fuel ratio, e.g., from a leaner value than the stoichiometric air/fuel ratio to the stoichiometric air/fuel ratio, there also can occur unstable rotation of the engine. Thus, in order to realize the benefits of a lean fuel air/fuel ratio, a method for monitoring engine operation stability is necessary.
An approach to monitoring combustion and computing a learned combustion stability value and applying the learned combustion stability value to control engine operation is described in U.S. Pat. No. 5,809,969, entitled "Method For Processing Crankshaft Speed Fluctuations For Control Applications," issued to Fiaschetti et al. on Sep. 22, 1998. The aforementioned issued U.S. patent is incorporated herein by reference. According to the methodology, engine speed is sensed for each expected firing of individual cylinders of the engine. The difference in engine speed for a selected cylinder firing and a cylinder firing occurring two cylinder firings earlier is determined to provide an expected acceleration value. The difference between successive expected acceleration values is computed. A learned combustion-related value is determined as a function of the difference in the successive learned acceleration values and is an indication of engine roughness. The learned combustion-related value is used to modify the fuel injection to an internal combustion engine, especially following a cold start, whereby hydrocarbon emissions can be reduced. This is accomplished by modifying a program target fuel injection value as a function of the learned combustion-related value so as to reduce the fuel injected into the engine by fuel injectors.